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Routing Information Protocol (RIP) RIP is a distance vector routing protocol Hop count is used as the metric for path selection If the hop count is greater than 15, the packet will be discarded By default, routing updates are broadcast every 30 seconds

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Routing Information Protocol (RIP) RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops in a path is 15. When a router receives a routing update that contains a new or changed entry, the metric value is increased by 1 to account for itself as a hop in the path. If this causes the metric to be incremented beyond 15, it is considered to be infinity and the network destination is considered unreachable.

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Routing Information Protocol (RIP) RIP sends routing-update messages at regular intervals. When a router receives a routing update that includes changes to an entry, it updates its routing table to reflect the new route. The received metric value for the path is increased by 1, and the source interface of the update is indicated as the next hop in the routing table. RIP routers maintain only the best route to a destination but can maintain multiple equal-cost paths to the destination.

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Configuring RIP The router rip command enables RIP as the routing protocol. The network command is then used to tell the router on which interfaces to run RIP. The routing process then associates specific interfaces with the network addresses and begins sending and receiving RIP updates on these interfaces.

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Distance Vector Routing Protocols There are several distance vector routing protocols in use today The most popular by a wide margin is Routing Information Protocol (RIP), followed by a Cisco proprietary protocol called Interior Gateway Routing Protocol (IGRP)

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Routing Loops Invalid updates will continue to loop until some process stops the looping. This condition is called count to infinity. Packets loop continuously around the network in spite of the fundamental fact that the destination network is down. These packets loop through the network because of wrong information in the routing tables.

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Routing Loops There are many different kinds of routing loops, but the simplest example of a routing loop occurs when one router believes the best path to a network is via a second router, and at the same time, the second router believes the best path to that network is through the first router

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Routing Loops – Count to Infinity Distance vector routing algorithms are self-correcting, but a routing loop problem can require a count to infinity. When a routing table update is received by a router in a distance vector network, the hop count for the updated route is incremented by one. Without countermeasures to stop the count to infinity process, the distance vector metric of hop count increments each time the packet passes through another router. To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number. With this approach, the routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value.

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Count to Infinity By defining “infinity” as a certain number of hops away (for example, 16 for RIP), the protocol is essentially saying, “any route with a hop count of 16 is unreachable” so packets to that network will be dropped instead of forwarded

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Routing Loops – Split-Horizon Another possible source for a routing loop occurs when incorrect information that has been sent back to a router contradicts the correct information that the router originally distributed. Split-horizon attempts to avoid this situation. If a routing update about a network arrives from a router, the router that receives the update cannot send information about that network back to the router that originally sent the update. Split-horizon thus reduces incorrect routing information and reduces routing overhead.

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The split horizon rule says that a router should not advertise a network through the interface from which the update came. Applying split horizon R3 advertises the 10.4.0.0 network to R2. R2 receives the information and updates its routing table. R2 then advertises the 10.4.0.0 network to R1 out S0/0/0. R2 does not advertise 10.4.0.0 to R3 out S0/0/1, because the route originated from that interface. R1 receives the information and updates its routing table. Because of split horizon, R1 also does not advertise the information about network 10.4.0.0 back to R2.

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Routing Loops – Split-Horizon The split horizon rule is based on the theory that it is not useful to send information about a route back in the direction from which it came. If router A receives an update from router D, it would not send that information back to router D.

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Routing Loops – Route Poisoning Route poisoning is used by various distance vector protocols in order to overcome large routing loops and offer explicit information when a subnet or network is not accessible. Route poisoning accomplishes this by setting the hop count to one more than the maximum. The maximum hop count for RIP is 15.

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Routing Loops – Route Poisoning Network 10.4.0.0 becomes unavailable due to a link failure. R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable. R2 processes that update. Because the metric is 16, R2 invalidates the routing entry in its routing table. R2 then sends the poison update to R1, indicating that route is unavailable, again by setting the metric value to 16. R1 processes the update and invalidates the routing entry for 10.4.0.0 in its routing table.

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Route poisoning speeds up the convergence process as the information about 10.4.0.0 spreads through the network more quickly than waiting for the hop count to reach "infinity".

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Routing Table Updates New routing tables are sent to neighboring routers on a regular basis (RIP updates occur every 30 seconds). However a triggered update is sent immediately in response to some change in the routing table. The router that detects a topology change immediately sends an update message to adjacent routers that, in turn, generate triggered updates notifying their adjacent neighbors of the change.

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Holddown Timers A count to infinity problem can be avoided by using holddown timers When a router receives an update from a neighbor indicating that a previously accessible network is now inaccessible, the router marks the route as inaccessible and starts a holddown timer

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Holddown Timers If at any time before the holddown timer expires an update is received from the same neighbor indicating that the network is again accessible, the router marks the network as accessible and removes the holddown timer. If an update arrives from a different neighboring router with a better metric than originally recorded for the network, the router marks the network as accessible and removes the holddown timer. If at any time before the holddown timer expires an update is received from a different neighboring router with a poorer metric, the update is ignored.

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Holddown Timers Holddown timers help prevent counting to infinity but also increase convergence time. The default holddown for RIP is 180 seconds. This will prevent any inferior route from being updated but may also prevent a valid alternative route from being installed. The holddown timer can be decreased to speed up convergence but should be done with caution. The ideal setting would be to set the timer just longer that the longest possible update time for the internetwork.

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Holddown Timers The ideal setting would be to set the timer just longer that the longest possible update time for the internetwork. Set the holddown timer for just over 120 second

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Examine Common IOS Show Commands 1. Examine show commands on a router Examine show commands in user EXEC mode show interfaces ۞ show interfaces serial 0/0/0 ۞ show version.... Examine show commands in privileged EXEC mode show arp ۞ show startup-config ۞ show running-config 2. Examine show commands on a switch Examine show commands that are common with a router Examine show commands unique to the switch show mac-address-table